Must agree with my illustrious (near) namesake. Leaving M5 any earlier than Taunton not advised particularly not via Bristol.
Depending on where in the West Midlands you start from, another, longer, route but with dual most of the way would be M40 - A34 - M3 _ A31 (Oxford, southampton, Poole) but unless you want a very leisurely trip do not recommentd the round 'down the middle' i.e. A429 - A350 via Cirencster and Chippenham. Beware spped camera about half way along the stretch from Taunton to Yeovil. If you want a lunch stop not quite to dorchester, there is a very large picnic area at the roundabout A303 - A37 about 5 miles before Yeovil - Catergate.